在Oracle 12c中将时间戳转换为日期

Question

I want to convert a given timestamp in such format: 2019-04-08 00:00:00.0 to a date in this format: 2019-04-08 .

I have already tried using:

SELECT TO_DATE('2019-04-20 00:00:00.0','YYYY-MM-DD') from dual; 

But I got prompted with:

ORA-01830: date format picture ends before converting entire input string

Answer 1

I think you may have some conceptual misunderstanding about how the TO_DATE function works, and also about how dates are processed by the DBMS.

YYY-MM-DD does not match the format of the actual string you're importing ( 2019-04-20 00:00:00.0 ) That's what the error is telling you. You must tell the TO_DATE function what to expect in the date string you input into it. You do that by means of the format string. if you don't specify a format string which matches the format you're actually going to supply, then the function will fail to process the string.

Next, you say you want to convert it "to a date in this format"...but this does not entirely make sense. TO_DATE converts a string into a variable of type DATETIME - ie a date object. A date object does not not exist in any particular format, it exists as an object. Internally it will store the date information in a way which is independent of any human-readable date format. The format relates entirely to the presentation of the date when seen as a string . Once you have a date object, you can then output the date in a particular format if you want to a human to be able to read it in the style that their culture is familiar with.

So, firstly to import your date string correctly as a date object, you can use an accurate format string, an also use TO_TIMESTAMP instead of TO_DATE so that it captures the sub-seconds value:

SELECT TO_TIMESTAMP('2019-04-20 00:00:00.0','YYYY-MM-DD HH24:MI:SS.FF5') from dual; 

If you run this in a console the SELECT will then automatically re-format that date object (the result of the TO_DATE function) into the default date format configured in your server / session.

However if you actually want to see it on screen in a particular format, you can explicitly say so - a sensible way is using the TO_CHAR function:

SELECT TO_CHAR(TO_TIMESTAMPT('2019-04-20 00:00:00.0','YYYY-MM-DD HH24:MI:SS.FF5'), 'YYYY-MM-DD') from dual;

The full list of format specifiers can be found here (and in other places online as well).

Live demo of the above here: https://dbfiddle.uk/?rdbms=oracle_18&fiddle=619d918ea73953e11b3150c6b560112c

Answer 2

Assuming the input is actual text, and not a real timestamp, you could try just truncating the text before you call TO_DATE :

WITH cte AS (
    SELECT '2019-04-20 00:00:00.0' AS ts FROM dual
)

SELECT TO_DATE(SUBSTR(ts, 1, 10), 'YYYY-MM-DD')
FROM cte;

If your input is an actual Oracle timestamp, and you want to convert it to a date, then you may use CAST :

SELECT CAST(ts AS DATE) dt
FROM cte;

Answer 3

Would CAST do any good?

I'm setting date format so that it displays time component, although it is 00:00:

SQL> alter session set nls_date_format = 'dd.mm.yyyy hh24:mi:ss';

Session altered.

SQL> select cast(timestamp '2019-04-20 00:00:00.0' as date) result from dual;

RESULT
-------------------
20.04.2019 00:00:00

Another format (without time component):

SQL> alter session set nls_date_format = 'dd.mm.yyyy';

Session altered.

SQL> select cast(timestamp '2019-04-20 00:00:00.0' as date) result from dual;

RESULT
----------
20.04.2019

SQL>

Or, using TO_CHAR function (so that session's date format doesn't matter):

SQL> select to_char(timestamp '2019-04-20 00:00:00.0', 'dd.mm.yyyy') result from dual;

RESULT
----------
20.04.2019

SQL>